Vγ9Vδ2 cells represent a major peripheral blood T cell subset in humans displaying a broad reactivity against microbial agents and tumors. They have the ability to simultaneously recognize and respond to phosphorylated non-peptide antigens (phosphoantigens, PAg), molecules found on a wide variety of pathogenic organisms and tumor cells in a HLA unrestricted fashion. Accordingly, Vγ9Vδ2 T cells are involved in tumor immune surveillance notably against carcinomas and hematologic malignancies.
Maintenance of lymphocyte population size is usually achieved by balancing the generation of new cells and clonal expansion with cell death. However, the homeostasis of Vγ9Vδ2T cells remain poorly understood. The size of the Vγ9Vδ2T cell pool is regulated by the availability of IL-15 and IL-7, and their homeostasis is maintained in competition with αβ T cells and NK cells. When both αβ and γδ cell types are adoptively transferred in equal numbers into TCRβ−/−/δ−/− mice, αβ T cells rapidly outgrow γδ T cells.
Thus, γδ T cells have a substantial disadvantage compared to αβ T cells during their expansion. Molecular pathways regulating the proliferation and homeostasis of γδ T cells are still not known. By comparison, it is well accepted that co-receptors positively or negatively regulate αβT cell activation, expansion and survival.
Among these co-receptors, molecules of the CD28:B7 family have a potent regulatory effect on TCR-mediated activation. Some of these co-receptors like PD-1 are also able to modulate Vγ9Vδ2 T cells proliferation.
BTLA, a recently described member of the CD28:B7 family structurally related to CTLA-4 and PD-1 is expressed by most lymphocytes. Its ligand, herpes virus entry mediator (HVEM), is a member of the TNF receptor superfamily expressed by T, B, and NK cells, dendritic cells and myeloid cells. BTLA-deficient mice exhibit a normal B and T cells development. However, mature lymphocytes from these mice, display higher frequencies of memory T cells and generate more memory-like responses. 20 Moreover, BTLA-deficient mice and in vitro observations obtained using agonist anti-BTLA antibodies and HVEM-Ig fusion proteins have evidenced BTLA as a negative modulator of immune responses against self- and allo-antigens and antigen independent homeostatic expansion of CD4+ and CD8+ T cells.
Indeed dysfunction of the BTLA-HVEM pathway is suspected to play a role in the pathogenesis of various auto-immune and neoplastic diseases, especially in the dysfunction of innate immunity in inflammatory diseases. Only few functional data about BTLA are available in Humans.
Cross-linking of BTLA with an agonistic mAb can inhibit αβ T cell proliferation and the production of IFN-γ and IL-10 in response to anti-CD3 stimulation.
Moreover, BTLA stimulation inhibits the function of both human melanoma-specific and CMV-specific T cells.
Among haematological malignancies, the inventors have previously shown that BTLA is expressed by neoplastic cells in B-CLL, but not in most B-non-Hodgkin's lymphomas (NHL), although various reactive immune cells of the lymphoma microenvironment are BTLA-positive.
The BTLA ligand, HVEM, displays frequent abnormalities in human B cell malignancies, especially in follicular lymphoma (FL), which harbor a high frequency of mutations in the TNFRSF14 (HVEM) gene. These mutations typically lead to the truncation of TNFRSF14 and are accompanied by the deletion of the wild type allele, which suggests a possible role for HVEM as a tumor suppressor gene.
The inventors have shown that BTLA expression is regulated not only during Vγ9Vδ2 T cell differentiation but also upon TCR-mediated activation. BTLA blockade improves TCR signaling. Furthermore, the inventors demonstrated that its interaction with HVEM negatively regulates TCR-independent and TCR-dependent Vγ9Vδ2 T cell proliferation, and that HVEM positive FL cells efficiently inhibited Vγ9Vδ2 T cell proliferation.